The present invention generally relates to digital communications and particularly relates to calibrating received signal measurements in a digital subscriber line device.
Digital Subscriber Line (DSL) systems use existing twisted-pair telephone lines to transport high-bandwidth data to subscribers. A number of forms of DSL technology have been developed, including Asymmetric DSL (ADSL), Symmetric DSL (SDSL), High bit-rate DSL (HDSL), and Very-high data-rate DSL (VDSL). Standardization efforts related to DSL are continuing, under the auspices of such groups as the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T).
Several versions of DSL, including ADSL2 and VDSL2, employ a line-coding method known as discrete multi-tone (DMT). In a DMT-based DSL system, the available transmission bandwidth is divided into a large number (e.g., 256) of separate sub-bands, each with a bandwidth of 4 kHz. Subcarriers corresponding to each of those sub-bands may be individually and independently modulated with data. Thus, one of several modulation schemes, ranging from Quadrature Phase-Shift Keying (QPSK) to 64-level Quadrature Amplitude Modulation (QAM-64), may be independently selected for each sub-carrier. A key advantage of DMT is that adaptive equalization is generally not required, since noise and interference characteristics do not vary substantially across a single 4-kHz sub-band. Instead, DMT-based DSL systems adapt to frequency-dependent noise, loss, and interference conditions by varying the modulation and/or coding on each individual sub-carrier. Thus, one sub-carrier may be modulated at a very high data rate, another at a lower data rate, while yet another may not be used at all.
DSL standards such as ADSL2 and VDSL2 specify that information regarding noise and loss conditions is reported by subscriber modems to the DSL management plane at initialization. (Initialization may occur at installation as well as after subsequent re-starts of the customer premises equipment). In particular, a logarithmic representation of the frequency-dependent loop transfer function (HLOG-ps), as well as a representation of the frequency-dependent quiet line noise (QLN-ps), are transmitted by the customer premises equipment to the remote modem during initialization. These reports essentially comprise vectors of values, wherein each element of the vectors corresponds to a sub-carrier/sub-band. The loop transfer function, HLOG-ps, equals the difference between the transmitted power spectral density (PSD)—the frequency-dependent distribution of the power transmitted by the remote modem—and the received PSD—the frequency-dependent distribution of the power actually received at the customer premises equipment (CPE). In other words, HLOG-ps represents the frequency-dependent loss experienced by the DSL signal as it propagates from the remote modem to the CPE modem. Thus, HLOG-ps data is collected while the remote modem is transmitting signals; calculating HLOG-ps requires knowing the transmitted power level. In contrast, QLN-ps represents the received PSD when no signal is transmitted by the far-end modem, i.e., the so-called QUIET state. These values are used by the remote modem in determining how to assign data and modulation schemes to the subcarriers.
To determine either HLOG-ps or QLN-ps, the receiver must measure, or estimate, the received PSD. Typically, the received PSD is estimated from frequency-selective voltage measurements made at the CPE modem. However, several problems limit the accuracy of those estimates and/or the usefulness of the resulting reports. First, the relationship between the voltage measurements and received power depends upon the input impedance of the CPE modem. This input impedance may vary from model to model, or from vendor to vendor. Furthermore, the voltage measurements may be performed by an application-specific integrated circuit (ASIC) supplied and/or programmed by one party, while the analog front-end circuitry (the design of which largely determines the input impedance) is supplied by another. Thus, the ASIC, which typically must calculate the PSD estimates, is typically unaware of the actual input impedance of the DSL device. Second, the voltage measurements depend not only on the transmitted power level and the nominal loss of the subscriber loop, but also on the relationship between the actual line impedance of the subscriber to the input impedance of the CPE modem.
Vendors typically sidestep these problems by simply programming their modems to measure the received voltage magnitude and divide the voltage measurements by a fixed factor that leads to reasonable results in a simple lab scenario. However, these proprietary approximation approaches lead to large variations of HLOG-ps and QLN-ps reports among different chipsets under real-world conditions.